1. Field of the Invention
The present invention relates to a disk-type brushless single-phase DC motor, and more particularly to a disk-type brushless single-phase DC motor including an armature coil attached to a stator yoke of a stator in such a fashion that it faces the lower surface of a rotor magnet having a plurality of alternating N and S poles, the armature coil having a closed loop structure provided with a plurality of uniformly spaced apexes, and cogging generating protrusions of a miniature size protruded from the stator yoke at positions spaced in a rotation direction of the rotor magnet from respective apexes of the armature coil by an angle corresponding to 1/4 of an angular width of one pole, thereby being capable of achieving an improvement in drive efficiency.
2. Description of the Prior Art
Generally, disk-type brushless single-phase DC motors are used in miniature fan motors for simple rotating appliances requiring no precise rotation, for example, office appliances such as computers.
Referring to FIG. 8 a disk-type brushless single-phase DC motor is illustrated which includes a housing 100 constituting a lower portion of the motor and serving to support elements of the motor, and a rotor 300 constituting an upper portion of the motor and arranged over the housing 100. The rotor 300 is rotatably coupled to the housing 100 by means of a shaft 200.
A multipolar rotor magnet 310 is mounted on the lower surface of the rotor 300 within the rotor 300. The multipolar rotor magnet 310 has a plurality of alternating N and S poles. The upper end of the shaft 200 is fixedly mounted to the central portion of the rotor 300. The shaft 200 extends downwardly through a bearing housing 110 upwardly protruded from the central portion of the housing 100 in such a fashion that it is rotatably supported by bearings mounted in the bearing housing 110. The upper end of the bearing housing 110 has a stepped structure in order to fixedly mount a stator 400 thereon.
The stator 400 mainly includes a printed circuit board 410, a stator yoke 420 laid on the printed circuit board 410, and armature coils 430 attached to the upper surface of the stator yoke 420 by means of an adhesive.
The driving of the disk-type brushless single-phase DC motor having the above mentioned configuration is achieved by a rotation of the rotor 300 carried out by an electromagnetic force generated between the armature coils 430 of the stator 400 and the rotor magnet 310.
This will be described in more detail. When single-phase current is supplied to the armature coils 430 via the printed circuit board 410, an electromagnetic force is generated in accordance with an interaction between the armature coils 430 and the rotor magnet 310, thereby generating a drive force. By this drive force, the rotor 300 rotates.
In this case, a coil torque 600 is generated between the armature coils 430 and the rotor magnet 310 by the electromagnetic force, as shown in FIG. 9. The coil torque exhibits a maximum value at the middle portion of each pole in the rotor magnet 310 and decreases gradually as the pole extends from the middle portion thereof to each lateral end thereof. The coil torque becomes zero at each lateral end of each pole, thereby causing the rotor 300 to stop.
The point, where the coil torque is zero, is called a "dead point". A cogging generating means is provided for a magnetic start-up at such a dead point.
Such a cogging generating means provides a cogging force serving as a load against the coil torque. Such a cogging force is adapted to increase the minimum coil torque while decreasing the maximum coil torque, thereby obtaining a substantially uniform torque. That is, a cogging torque, which has a waveform 700 in FIG. 9, is generated simultaneously with the generation of the coil torque, which has a waveform 600 in FIG. 9, thereby obtaining an ideal resultant torque which has a waveform 800 in FIG. 9. The cogging torque, which serves as a load against the coil torque, has an output level inversely proportional to the output level of the coil torque, thereby reducing the variation in the resultant torque. As a result, the motor can drive stably.
A variety of motors provided with such a cogging means have been proposed in, for example, U.S. Pat. No. 4,620,139, U.S. Pat. No. 4,757,222, and Japanese Patent Publication No. Heisei 7-213041. The cogging means disclosed in the patents generates an appropriate cogging torque serving as a load against a coil torque. In accordance with a combination of the cogging torque and coil torque, an ideal resultant torque is obtained.
Meanwhile, the coil torque and cogging torque exhibit a phase difference corresponding to about 1/4 of the pole width therebetween. Accordingly, the cogging generating means is arranged at a position where the coil torque is zero during a rotation of the rotor.
In U.S. Pat. Nos. 4,620,139 and 4,757,222, as shown in FIG. 10, the cogging generating means comprises iron cores 440 coupled to or fitted to the stator yoke 420 in such a fashion that they are protruded from the stator yoke 420 toward the rotor magnet 310. Alternatively, the cogging generating means may be provided by cutting out opposite arc-shaped peripheral portions of the stator yoke 420, as shown in FIG. 11. In this case, the cogging generating means comprises arc-shaped cutouts 450. In both cases, however, there is a problem in that it is difficult to determine an accurate position of the cogging generating means because the position of the cogging generating means has an inseparable relation with the attachment position of the armature coil.
In both cases, an accurate position for installing the cogging generating means thereon is first determined with respect to each armature coil 430 attached to the stator coil 420. The coupling or fitting of the iron core 440 to the stator coil 420 is carried out at the determined position. In the case wherein the arc-shaped cutouts are used as the cogging generating means, those cutouts are formed at positions determined as above, respectively. However, the position determination for the cogging generating means is very difficult unless a jig is used.
Since a pair of armature coils 430 are practically attached to the stator yoke 420 in such a fashion that they are opposite to each other, a great loss of magnetic force occurs at stator yoke portions where no armature coil is attached, thereby generating a reduced coil torque. As a result, an insufficient drive torque is obtained. This results in a considerable performance degradation.
On the other hand, in Japanese Patent Publication No. Heisei 7-213041, the cogging generating means comprises magnetic members 460 as shown in FIG. 12. Each magnetic member 460 is positioned at an angle .theta. (0&lt;.theta.&lt;.pi., where .pi. is an electrical angle and equal to 180.degree.) from the dead point. In particular, the magnetic members have a screw construction so that they also serve as a fixing means for fixing the printed circuit board 410 and stator yoke 420 to each other.
In this case, however, the screw members preferentially have the function for fixing the printed circuit board 410 and stator yoke 420 to each other over the cogging generating function. For this reason, after the printed circuit board 410 and stator yoke 420 are fixed to each other, the screw members may have different gaps with respect to the associated rotor magnets 310, respectively. As a result, the cogging torque generated by the cogging generating means may vary for different screw members.
In other words, a difference in fastening degrees of the screw members result in a variation in the magnetic force generated by the rotor magnets 310, thereby generating an instable drive torque.
In the case of a miniature motor, furthermore, it is impossible to fasten the magnetic members 460 having a very small size unless a specific tool is used. Moreover, it is also impossible to adjust the fastening degree of the magnetic members 460. Consequently, it is impossible to practically apply such a construction to miniature motors.